The correlation of the Young's modulus and crystal orientation of iron is extremely strong. For example, the <111> orientation Young's modulus ideally is over 280 GPa, while the <110> orientation Young's modulus is about 220 GPa. On the other hand, the <100> orientation Young's modulus is about 130 GPa. The Young's modulus changes according to the crystal orientation. Further, when the crystal orientation of the steel material does not have orientation in any specific direction, that is, the texture is random, the Young's modulus of the steel sheet is about 205 GPa.
Up to now, a large number of technologies have been proposed regarding steel sheets controlling the texture to raise the Young's modulus in a direction perpendicular to the rolling direction (referred to as the “transverse direction”). Further, for technology for simultaneously raising the rolling direction and transverse direction Young's modulus of steel sheet, for example, Japanese Patent Publication (A) No. 4-147917 proposes a method of production of steel plate not only rolling in a certain direction, but also rolling in a direction perpendicular to this. This method of changing the direction of rolling in the middle can be performed relatively simply in the process of rolling steel plate.
However, even in the case of producing steel plate, depending on the width and length of the steel plate, it is sometimes necessary to make the rolling direction fixed. Further, in particular in the case of thin-gauge steel sheet, the sheet is often produced by the continuous hot rolling process of continuously rolling a steel slab to obtain a steel strip, so technology changing the rolling direction in the middle is not practical. Furthermore, the width of the thin-gauge steel sheet produced by the continuous hot rolling process is at most about 2 m. For this reason, for example, to apply a high Young's modulus steel sheet to a building material or other long member of over 2 m, it was necessary to raise the rolling direction Young's modulus.
To meet such demands, some of the inventors proposed the method of giving shear strain to the surface layer of a steel sheet part to raise the rolling direction Young's modulus of the surface layer part (for example, Japanese Patent Publication (A) No. 2005-273001, International Patent Publication No. 06-011503, Japanese Patent Publication (A) No. 2007-46146, and Japanese Patent Publication (A) No. 2007-146275).
The steel sheets obtained by the methods proposed in these patent documents have textures increasing the rolling direction Young's modulus at the surface layer part. For this reason, these steel sheets have high Young's moduli of the surface layer parts and have Young's moduli measured by the vibration method of over 230 GPa.
One method of measurement of the Young's modulus, that is, the vibration method, gives bending deformation to the steel sheet while changing the frequency, finds the frequency at which resonance occurs, and converts this to the Young's modulus. The Young's modulus measured by this method is also called the “dynamic Young's modulus”. This is the Young's modulus obtained at the time of bending deformation. The contribution of the surface layer part with the large bending moment is great.
However, for example, when a load is applied to long beams or columns or other building materials or structural members of automobiles such as pillars or support members or other such long frame members, the stress acting on these is tensile stress and compressive stress and not bending stress. Further, automobile support members require a high impact absorption energy ability when receiving compressive deformation from the viewpoint of impact safety. For this reason, to improve the impact absorption energy of the member, it is necessary to secure the rigidity with respect to the tensile stress and compressive stress. In the face of such demands, it is effective to raise the Young's modulus in the longitudinal direction of the member with respect to the tensile stress and compressive stress.
Therefore, for the Young's modulus of the member on which this tensile stress and compressive stress act, it is extremely important to raise the Young's modulus measured by not the vibration method, but the static tension method, that is, the static Young's modulus. The static Young's modulus is the Young's modulus found from the inclination at the elastic deformation region of the stress-strain curve obtained at the time of the tensile test. It is the Young's modulus of the material as a whole determined by only the ratio of the thickness of the high Young's modulus layer and low layer.
To raise the rolling direction static Young's modulus, it is necessary to control the texture from the surface layer to a location deep in the plate thickness direction. Note that control of the texture of the entire sheet thickness from the surface layer to the sheet thickness center location is more preferable.
However, in the method proposed in these patent documents, it was difficult to introduce shear strain up to the center part of the plate thickness at the time of rolling. Further, depending on the ingredients and production conditions, in the texture of the sheet thickness center part, there is a possibility of a formation of orientation lowering the rolling direction Young's modulus.
For this reason, while the Young's modulus measured by the vibration method can be raised to 230 GPa or more, the Young's modulus measured by the static tension method is not necessarily high. That is, there has never been steel sheet with a rolling direction Young's modulus measured by the static tension method of 220 GPa or more.